The present invention relates to a valve assembly for use in pressurized fluid systems and particularly relates to a check valve assembly for pumps used in liquid chromatography systems.
In liquid chromatography systems, reciprocating pumps are generally used to pump chromatographic solvents through a liquid chromatography column. A specific example of a pump, particularly a dual piston reciprocating pump assembly, useful for this purpose is described and illustrated in copending U.S. patent application Ser. No. 820,592 filed Aug. 1, 1977, now U.S. Pat. No. 4,173,437 of common assignee herewith. In pumping systems of that and other types, a retracting piston draws liquid into a vacated piston chamber through a pair of open check valves disposed in a liquid inlet passage to the chamber. A pair of check valves is also disposed in a liquid output conduit from the piston chamber. It will be appreciated that the pairs of inlet and outlet valves are normally in open and closed positions respectively in their respective conduits when the piston is vacating the piston chamber. When the piston is pumping, i.e., displaced into its chamber, the pairs of inlet and outlet valves are in closed and opened positions respectively. For further description of the aforementioned pump assembly used in a chromatographic system, reference is made to copending U.S. patent application Ser. No. 820,592 filed Aug. 1, 1977, now U.S. Pat. No. 4,173,437 of common assignee herewith, the disclosure of which application is incorporated herein by reference as though fully set forth herein.
It will also be appreciated that, in liquid chromatographic systems, a wide variety of liquids are pumped through the systems and these liquids are frequently highly corrosive in nature. For example, acetic acid and nitric acid are liquids often used in liquid chromatographic systems and these liquids are extremely corrosive. As a consequence, liquid chromatographic systems are manufactured using materials which are inert to most liquids passed through these systems.
Various materials have been proposed and utilized in constructing the valve assemblies for the pumps used in liquid chromatographic systems. For example, check valves utilizing synthetic sapphire balls and stainless steel valve seats have been manufactured and used. It has been observed, however, that the chamfer of the stainless steel valve seat, which cooperates with the sapphire ball to form the seal in the valve closed condition, frequently becomes spotted, pitted or dimpled in use. Also, substantial chips and cavities have been formed in these stainless steel valve seats. Under the substantial liquid pressures utilized in liquid chromatographic systems, on the order of 6000 psi, and because of these degradations to the valve seats, leaks deleterious to the operation of the systems have occurred. While the exact causes of the degradation of the stainless steel valve seats have not been identified with certainty, it is believed that metal fatigue under the constant seating and unseating action of the sapphire ball at high pressures as well as possible generation of an electrostatic field which could build up a static charge on the metal seat may cause the pitting and dimpling and the chipping respectively. Grit and dirt in the flowing liquid, as well as the corrosive effects of some of the solvents used in flowing systems, also cannot be ruled out as a source of the failure of the valve seats to form a liquid tight seal with the sapphire ball.
It is also known to utilize a synthetic sapphire ball in conjunction with an annular synthetic sapphire seat. Since sapphire is a non-conductive material, chipping which may be caused by the generation of an electrostatic charge on the conductive stainless steel seat, would be eliminated. However, a valve seat formed of synthetic sapphire material is highly brittle and prone to break. Also, it is extremely difficult to obtain a round opening in a sapphire valve seat and which opening will accurately and repeatedly seal with the mating sapphire ball. Thus, sapphire seats frequently do not seal with their mating balls as well as they theoretically should and further are subject to chipping if hard material is carried into the valve assembly by the liquid stream. Further, because synthetic sapphire is a brittle material, it cannot accept pressure without a resilient mounting between it and its housings. Thus, it is necessary to seal the sapphire seat to its surrounding housing, for example by using "O" rings. Consequently, use of sapphire seats as part of check valve assemblies in liquid chromatographic systems renders the system expensive and difficult to manufacture.